In their basic configuration, hydraulic bearings of this kind usually comprise a rubber element as a support spring in combination with a hydraulic damper. The hydraulic damper, in turn, comprises a hydraulic volume which is partitioned by a partition wall into a work chamber and a compensating chamber. The partition wall is provided with at least one throttle channel. With the aid of such hydraulic bearings, which are disposed between the vehicle motor and the chassis, it is intended, on the one hand, to prevent that motor vibrations are transmitted to the chassis and, on the other hand, it is intended that the shaking, which results during driving operation, cannot reach the motor from the chassis or reach the motor only dampened.
Furthermore, it is prevented that acoustic vibrations can reach the chassis and especially acoustic vibrations in the form of structure-borne noise. For this purpose, the partition wall, which is disposed between the work chamber and the compensating chamber, includes a further opening which is covered by a flexible to rigid component. This component is usually characterized as a membrane and is axially moveable over a clear path. In addition to functioning as a vibration-damping support construction for motor vehicle engines, such hydraulic bearings function to filter acoustic vibrations.
These acoustic vibrations are of higher frequencies but lower amplitudes. To take up these acoustic vibrations, this membrane-like component is provided with lattice plates on respective sides thereof which function as displacement-limiting end stops. The membrane-like component is also characterized as an acoustic membrane.
Two principally different constructions are used in hydraulic bearings as components for decoupling or damping acoustic vibrations, namely:                (1) loose, axially-moveable plates in the manner of floating pistons with these plates not being laterally clamped; and,        (2) flexible laterally-clamped membranes.        
Plates, which are not laterally clamped, present the following disadvantages:    (a) A slit is provided between the edge of the plate and the edge of the partition wall recess holding the edge. Depending upon the position of the plate, the slit can be larger or smaller. This slit defines a bypass which is connected in parallel to the damping channel. Because of its uncontrollable size, the bypass influences the damping characteristics of the bearing in a nonreproducible manner.    (b) The plates act as floating pistons which are effective as structure-borne noise filters only when the plate does not lie on the one or the other of the displacement limits. Lying against the displacement limits is not only present when the acoustic amplitude is greater than or equal to the clear path of the plate but also when a quasi steady state uneven pressure distribution is present on both sides of the plate because of the flow resistance effective in the damping channel. More specifically, the plates are effective as structure-borne noise filters only under the above-mentioned limiting preconditions.
The invention is concerned with the use of membranes according to point (2) above.
U.S. Pat. No. 4,700,931 discloses an elastic bearing for supporting engines, machine assemblies, et cetera. A specifically configured decoupling membrane, which is reinforced at the periphery, is provided for improving the acoustic characteristics. This decoupling membrane has an edge which is clamped into a partition plate cutout and can move between limiting lattices within a clear path on both sides. Neither a radial prestressing is adjusted or has to be adjusted nor is a matching to a specific natural frequency or resonance frequency required so that it remains unclear, in detail, in which frequency range an acoustic improvement could function and if so, how.
U.S. Pat. No. 5,344,127 discloses a switchable hydraulic bearing which includes an annularly-shaped acoustic membrane whose reinforced edge is axially clamped at a partition plate cutout (see FIG. 5 of this patent). It is not explained in detail as to whether a matching of the membrane to specific frequencies or frequency ranges takes place.
German patent publication 196 50 230 likewise discloses a hydraulic bearing with an acoustic filter component. This filter component includes an additional compensating chamber which is closed off against the hydraulic chamber by a throttle membrane and is closed off relative to the (first) compensating chamber by an additional (second) compensating chamber membrane. A flow channel connects the second compensating chamber to the work chamber or to the first compensating chamber. With this acoustic filter component, it is intended that the increase of the dynamic spring rate to a second stiffness level is prevented. This increase is construction dependent in simple hydraulic bearings.